It has been known that exposure to very bright sunlight can cause damage to the retina. Thus, sunglasses should not only reduce the level of visible light entering the eye but also possess ultraviolet (UV) light blocking capabilities. Surprisingly, there is no real consensus on what is an acceptable level of UV exposure. Some may need sunglasses while others do not. Those that wear sunglasses can enjoy some protection, while others, if the warnings are correct, may sustain some damage, however slight.
It has also been known that art and photographic works deteriorate when exposed for long periods to sunlight or even to fluorescent light. Again, there are no established standards concerning the time and level of exposure. Also, different dyes and pigments held in a variety of different solvents and binders in different works will be effected differently.
Various approaches have been suggested to eliminate or reduce the unwanted effects of UV light in these applications. Certain resins and plastics have been developed which, either due to their own structure and composition, or when used as a host matrix for other substances which have the necessary properties, will block UV radiation by absorption. The plastic materials may be used in various ways such as a glazing material, as a laminated element in a multi-component glazing, or as an optical element in ophthalmic applications.
The typical performance of two different plastic materials, produced by the Rohm and Hass Company, Philadelphia, Pa., and designated UF 3 and UF 4, is shown in FIG. 1. The longer wavelength blocking material UF 3 (curve 20) imparts a slight yellow tint. The shorter wavelength blocking material UF 4 (curve 22) imparts no perceptible tint to the transmitted light.
Resin formulations are typically based on silicon siloxane resins containing appropriate additives. They are designed to be coated on glass or UV transparent plastic by spinning or roll coating and then are heat cured. The performance of such products compares with the plastic materials UF 3 and 4.
Certain special glass compositions have also been developed which through the use of oxides of cerium and other materials will effectively block UV radiation. The use of these glass compositions seems to be confined to space and military applications, such as protective covers for silicon solar cell arrays. These compositions apparently have not been used extensively for corrective spectacles, sunglasses, or as protective glazings for art and photographic works.
The present invention relates to a special thin film multilayer structure which is dispersive and absorbing in the near ultraviolet region of the spectrum. The structure may be deposited as a coating in a single operation. It provides antireflection and UV blocking properties.
It may be applied to a wide range of glasses and plastics, and it does not require any special formulations of these materials. It offers a more economical means of providing both UV and anti-glare protection than if these properties were realized by separate process steps or separate structural elements.
The present invention makes use of the fact that the refractive index and absorption coefficient of certain dielectric materials rises rapidly at wavelengths shorter than 450 nanometers (nm). It is an improvement over the type of multilayer coating described by Rock in U.S. Pat. No. 3,432,225 and Sulzbach in U.S. Pat. No. 3,565,509, both of which are hereby incorporated by reference. The coatings disclosed in those patents use two or more relatively thin films to replace the quarter wave film next to the substrate in the classical three layer "Quarter, Half, Quarter" system. This system was originally described by Lockhart and King in "Three-Layered Reflection-Reducing Coatings", J. Opt. Soc. Am., Vol. 37, pp. 689-94 (1947), which is also incorporated herein by reference.
The Lockhart and King structure comprises three films in which the outer film has an optical thickness of approximately one-quarter wavelength in the visible spectrum and a refractive index (N) less than that of a glass substrate (N=1.52). The second film has an optical thickness of one-half wavelength and a relatively high refractive index, e.g. on the order of 2.10. The third or inner film has an optical thickness of one-quarter wavelength and a refractive index less than that of the second or half-wave film but greater than the glass substrate, e.g., on which the structure is deposited.
The Rock structure, described in U.S. Pat. No. 3,432,225, replaces the inner film adjacent the substrate of the Lockhart and King structure with two films. The refractive index of the innermost film of the two films is equal to that of the half-wave film. The other film has a refractive index equal to the low index outer film. The thickness relationship of the films is altered slightly to optimize performance. Specifically, the optical thickness of the two outermost films remain about the same, while the optical thickness of the two innermost films are each about one-eighth of a wavelength at 520 nm (the design wavelength). The two-film substitution is effective not only in simulating the index of the film that has been replaced but also provides an additional reflecting boundary which extends the effective spectral range of the structure. The total optical thickness of the Lockhart and King, and Rock structures are very nearly the same.